1. Field of the Invention
The present invention relates to a pneumatic tire which includes a tread rubber capable of discharging electrostatic charges of the vehicle body to the road surface. It also relates to method and apparatus for producing unvulcanized tread rubbers for such tires.
2. Description of the Related Art
As known widely, electrostatic charges generated in the vehicle body of passenger cars, trucks or busses cause not only electroshock, but also occasional failure of the vehicle functions due to spark discharge. Such problems do not arise when the tires are comprised of a rubber which contains a large amount of carbon black because the tire has a sufficiently is high conductivity, and the electrostatic charges generated in the vehicle body can be discharged to the road surface through the tire.
There is an increasing demand for tires capable of minimizing fuel consumption of vehicles not only in view of economics, but also from environmental considerations. In order to cope with such demand, it would be desirable to employ low rolling resistance tires in which a large part of carbon black in the tire rubber is replaced with silica. However, by decreasing the amount of carbon black and increasing the amount of silica, it is inevitable that the electric resistance value of the tread rubber increases, making it difficult to discharge electrostatic charges of the vehicle body to the road surface.
In order to discharge electrostatic charges of the vehicle body to the road surface, a discharge means typically in the form of a metal strip may be directly connected to the conductive material of the vehicle body. However, such metal strip rubs or collides with the road surface and has to be exchanged whenever it is worn or broken; otherwise, the above-mentioned problems arising from the electrostatic charges of the vehicle body would be inevitable.
In order to overcome the above-mentioned disadvantages, there has been proposed, for example in JP-08-34204 A, a tire having an extruded tread rubber, in which a tread rubber comprises a conventional high conductive rubber containing carbon black, which is arranged on the center region, and a low conductive rubber containing silica, which is arranged at the remaining regions. According to this proposal, the high conductive rubber region in the tread rubber is connected to an ordinary base rubber containing carbon black, to ensure that the tire exhibits the desired electrostatic discharge function.
However, rubber-reinforcing effect achieved by silica is lower than that by carbon black, and there is a tendency that the tread rubber undergoes non-uniform wear as running of the tire proceeds. More specifically, the silica-blended low conductive rubbers on both sides of the center region exhibit a large amount of wear as compared to the center region comprised of high conductive rubber which contains carbon black. As a result, there tends to be caused a level difference, between the high conductive rubber portion and the low conductive rubber portions on both sides thereof. Such a level difference becomes significant as running proceeds and spreads in the width direction of the tread, causing a marked irregular wear and thereby reducing the service life of the tire.
Furthermore, it has been confirmed that the tread rubber tends to be twisted due to the slip angle which is applied to the tread rubber during cornering of the vehicle, and the high conductive rubber in the center region of the tread projects outwards and apply a large tearing force to the junction between the high conductive rubber and the silica-blended rubber adjacent to the high conductive rubber. Such tearing force tends to cause cracks in the silica-blended rubber which does not provide a sufficient rubber-reinforcing effect. Furthermore, the high conductive rubber occupies a large volume in the tread rubber which results in a degraded rolling resistance property. This shows that the pneumatic tires of this kind requires further improvement.